[css-images] Wrong <color-stop-list> syntax

https://drafts.csswg.org/css-images-3/#color-stop-syntax

```
<color-stop-list> = <color-stop>{2,}
<color-stop> = <color> <length-percentage>?
```

According to this syntax comma between `<color-stop>` is not needed, but comma is required. Looks like `#` is missed:

```
<color-stop-list> = <color-stop>#{2,}
<color-stop> = <color> <length-percentage>?
```
